Curable thermoset coating compositions are widely used in the coatings art. They are often used as topcoats in the automotive and industrial coatings industry. Such topcoats may be basecoats, clearcoats, or mixtures thereof. Color-plus-clear composite coatings are particularly useful as topcoats where exceptional gloss, depth of color, distinctness of image, or special metallic effect is desired. The automotive industry has made extensive use of these coatings for automotive body panels.
Color-plus-clear composite coatings, however, require an extremely high degree of clarity in the clearcoat to achieve the desired visual effect. High-gloss coatings also require a low degree of visual aberrations at the surface of the coating in order to achieve the desired visual effect such as high distinctness of image (DOI). Finally, such composite coatings must also simultaneously provide a desirable balance of finished film properties such as durability, hardness, flexibility, and resistance to environmental etch, scratching, marring, solvents, and/or acids.
In order to obtain the extremely smooth finishes that are generally required in the coatings industry, coating compositions must exhibit good flow before curing. Good flow is observed when the coating composition is fluid enough at some point after it is applied to the substrate and before it cures to a hard film to take on a smooth appearance. Some coating compositions exhibit good flow immediately upon application and others exhibit good flow only after the application of elevated temperatures.
One way to impart fluid characteristics and good flow to a coating composition is to incorporate volatile organic solvents into the composition. These solvents provide the desired fluidity and flow during the coating process, but evaporate upon exposure to elevated curing temperatures, leaving only the coating components behind.
However, the use of such solvents increases the volatile organic content (VOC) of the coating composition. Because of the adverse impact that volatile organic solvents may have on the environment, many government regulations impose limitations on the amount of volatile solvent that can be used. Increasing the percentage of nonvolatile components (% NV) of a coating composition or decreasing the VOC provides a competitive advantage with respect to environmental concerns, air permitting requirements, and cost.
Prior art attempts to improve the VOC of polymers and coating compositions have generally focused on the removal of volatile organic solvents from polymers by methods such as vacuum distillation. However, such techniques have significant disadvantages. First, they generally require the use of more energy and labor that leads to higher costs. Increased costs also result from the disposal of removed solvent. Finally, the viscosity of the stripped polymer often creates processing and manufacturing challenges.
Although good flow and decreased VOC are highly desired, such improvements must not be obtained at the cost of environmental durability and weatherability. It will be appreciated that the prior art has long attempted to provide improvements in the environmental durability as measured by testing apparatus such as the QUV cabinet made by Q-Panel Lab Products of Cleveland, Ohio, and WOM weatherometers made by Atlas Material Testing Solutions of Chicago, Ill., and the like. Such improvements are highly desired since they immediately translate to longer and better performance lifetimes for applied coatings.
Thus, it would desirable if improvements in environmental durability could be obtained in individual components of curable coating compositions. In another aspect, it would be desirable if improvements in flow and VOC could be obtained with simultaneous improvements in environmental durability and weatherability.